It is known to provide a battery-powered vehicle, often an industrial vehicle such as a forklift truck or, more generally, vehicles driven by battery power, with a direct-current series-wound electric motor having an armature connected in series with the field winding of the motor.
To control the motor, a main thyristor is connected in series with the motor across the battery and is triggered into operation by a pulse generator, thereby effecting so-called pulse control of the vehicle speed.
More specifically, a resistor is connected in series with the direct-current motor across the battery and can be bridged by a switch. Such circuitry is commonly available for electronic pulse control of an electrically powered vehicle using a battery as the source of electrical energy.
For the braking of the vehicle, it is common practice to use the motor as a generator and to dissipate the electrical energy which is thereby produced. For maximum economy of operation, it is advantageous to return the electrical energy gained during motor braking, i.e. the electrical energy produced by the motor when the latter is operated as a generator, to the battery.
The resistor connected in series between the motor and the battery is usually dimensioned so that self-excitation of the motor/generator will be permitted.
The capacitor is included in a commutating network which is charged after quenching of the main thyristor via the resistor, the latter being traversed, during the braking operation, by the excitation current, and through a second thyristor, i.e., a so-called quenching thyristor.
In prior-art circuits of this type, the resistor connected in series with the motor and through which the excitation current flows has a low resistance value, i.e. is a low-ohmic resistor. The capacitor of the commutating device is thus able to be charged relatively rapidly through this resistor so that no difficulties are encountered because of a lag in the capacitor charging. However, since this resistor has a small resistance value, a relatively large current flows through this resistor and substantial amounts of energy are dissipated therein as heat, thereby reducing the economy of the system. An additional problem is the abstraction or removal of the large quantity of heat thus produced.